The invention is adapted to be used in a control system of the kind disclosed in U.S. patent application Ser. No. 582,470, filed Sept. 14, 1990, entitled "Automatic Transaxle Control System For An Automotive Vehicle", which is assigned to the assignee of this invention.
The control system shown in the aforementioned application has features that are common to the control system of U.S. Pat. No. 4,509,389 which also is assigned to the assignee of this invention.
The '389 patent discloses a control system for a multiple ratio transmission having clutch and brake servos that are operated sequentially to effect plural torque transfer paths between an engine and the driven member of the vehicle. It includes a valve circuit having a main regulator valve for establishing the optimum pressure level for the servos under varying operating conditions and driver demands. The valve circuit includes multiple shift valves for sequentially actuating and releasing the servos to effect torque ratio changes. The shift valves respond to a fluid pressure governor signal and to a manual throttle valve output signal, the latter being representative of engine torque, and the former being representative of vehicle speed.
The disclosure of the aforementioned patent application differs from the control circuit of the '389 patent because it does not include a manually operable mechanical throttle valve system nor a fluid pressure governor driven by the driven member of the transmission. Instead, the shift valves are controlled by solenoid operated valves which trigger shifting movement of the shift valves between upshift and downshift positions as the solenoid valves respond to a microprocessor electronic control circuit. Further, the mechanical throttle valve system has been replaced by a throttle valve solenoid valve which also responds to signals developed by the microprocessor control circuit.
The main regulator valve of the control system of the aforementioned patent application, as in the case of the system of the '389 patent, responds to the torque signal to develop the desired circuit pressure. If the throttle valve solenoid pressure signal should not be available because of a loss of control voltage or because of a valve malfunction, the line pressure maintained by the main regulator valve may be insufficient to satisfy the capacity demands of the clutch and brake servos. Furthermore, the output signals from the shift solenoid valves would be unavailable for appropriately controlling the transmission ratios if those valves fail to function properly or if a voltage failure occurs.